Toronto’s Urban Forest

Toronto’s urban forest is a precious relic of what must have been, before the settlement of Ontario by European farmers, a vast, unbroken expanse of forest with about sixty species of trees native to Southwestern Ontario: cherries (chokecherry, black cherry, pin cherry), nut trees (chestnut, butternut, butternut hickory, walnut), berries (hackberry, serviceberry, red mulberry), and the more familiar oak, elm, birch, beech, ironwood, and the conifers such as cedar, spruce, tamarack (which is also deciduous), fir, pine and hemlock, which can live up to 600 years. (For an exhaustive list, click here.)

One common tree, especially in newer neighbourhoods, is the honey locust, which is well suited to urban environments because it is resistant to salt pollution, drought and various pests.

Some trees have been decimated by disease. Only young and medium-size elms can be found today. This once widespread urban shade tree has been destroyed by Dutch Elm disease. The butternut tree is now listed as endangered because of a canker disease. But many of us can never forget the devastation of some 28,000 trees in Toronto and Vaughan caused by the Asian longhorn beetle that required almost a decade to bring under control.

Up to 70% of the trees and canopy of the urban forests in Canada could be destroyed by this invasive insect. Another invasive insect from Asia, the emerald ash borer, first appearing in Ontario in 2002, is 99% effective in killing ash trees because it has no natural enemies. There are about 860,000 ash trees in Toronto, tens of thousands of which have already died. The green and white ash trees account for 8.9% of all trees.

Another modern threat to the urban forest is invasive species. For example, the Russian olive, native to Central Asia, Iran and Turkey, was introduced to North America in the 19th century, and has since outcompeted native trees. Yet another threat to native trees is hybridization, that is, interbreeding of species that crowd out the original native tree. Butternut is a case in point, being frequently cross-bred with various non-native walnut species. It may surprise you that the venerable Norway maple is an invasive tree and accounts for 6.5% of all trees in Toronto, and in some areas, it dominates the canopy. Its numbers, however, are declining as it is slowly replaced by native trees.

One last threat to the urban forest is fragmentation of habitat. Trees and the animals and insects that live in them or depend on them as a food source need a certain unbroken, continuous area of land to be sustainable. Naturally, with the haphazard and poorly regulated growth of almost all urban agglomerations in the world, it is actually a miracle that so much has survived. In Toronto, the ravines with their creeks have been natural, continuous territories for the urban forest. They have also been prime real estate for developers, who tout the “ravine views” of properties built at their edges.

With the growth of science, the public support for measures to ameliorate climate change, and the general awareness of conservation, has come a veritable revolution in how we understand and value the urban forest.

We owe the very existence of the urban forest, as well as parks, including conservation areas and national parks, to Romanticism, a key principle of which was that cities corrupt human beings and are an unnatural environment. We should live close to nature. These ideas lie behind the drive to set aside parkland, including university campuses, as way of providing at least some taste of nature even in paved-over, crowded, polluted cities. It is also true that many of these parks in cities were geometrically designed and manicured; nevertheless, many city-dwellers have been grateful for their open spaces and greenery.

Currently, the emphasis is on viewing forests, urban or otherwise, as parts of the web of life, the earth’s biome, with its intricate and often unsuspected interconnections. A good illustration of this view is the discovery of how the Amazon rainforest relies on the Sahara Desert, hardly an obvious connection. Winds in the Sahara, especially those passing over “the Bodélé Depression in Chad, an ancient lake bed where rock minerals composed of dead microorganisms are loaded with phosphorus.” The winds transport the phosphorous particles aloft into the stratosphere where they cross the Atlantic Ocean on a 1600-mile journey to the Amazon rainforest, where they are deposited. It is estimated that 182 million tons of dust are transported to South America annually, 27.7 million tons of which are deposited on the Amazon rainforest, the equivalent of 104,908 semi trucks, according to data gathered from NASA’s CALIPSO Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation.
The phosphorus is essentially for maintaining the rich fertility of the rainforest.

Another example comes from David Suzuki. In October, millions of salmon and other fish swim up inland streams, where thousands of bears await their arrival. Each bear captures hundreds of kilos of fish from the streams. So rich is the supply that bears may eat only the delicacies of the fish, leaving behind as much as 95% of the fish to rot in the forest. The fish are rich in nitrogen, an element that is an excellent fertilizer. Studies have shown that some trees on the British Columbia coast receive 80% of their nitrogen from ocean fish such as salmon.

A last illustration of the current way of integrating science, the humanities (here, history and social science) and art (consilience) is a cautionary tale about forests, based on the reasons for the decline of the Mayan Empire. In about 900 AD the Mayans, after dominating a large area of Mexico and Guatemala for over a millennium, entered a period of sharp decline. Mayan buildings such as temples, pyramids and observatories contained extensive walls that were traditionally covered with murals that depicted the reigns of various kings and religious myths.

Making the plaster for these murals required a very high temperature to melt limestone, a key ingredient. In the absence of coal, the Mayans had to burn twenty trees to produce one square meter of plaster. This led to deforestation, which in turn raised the air temperature a few degrees, enough to exacerbate the drought and cause farmers to try to increase production. This over-farming depleted the land further, reducing the food supply just at a time when it became impossible to store water for the densely populated cities. This chain of events demonstrates the value of forests for their ability to prevent soil erosion, store water, and provide a canopy that keeps the land cooler.

In general, although recreation and the appreciation of the beauty of forests are still important factors, there has been a general shift toward appreciating the practical advantages of the urban forest: the cooling and warming caused by the tree canopy; the prevention of soil erosion; the sequestration of carbon that would otherwise be in the atmosphere contributing to global warming; the forests as promoters of biodiversity, and so on. Even in the case of recreation, some studies indicate that urban forests decrease stress and, consequently, cardiovascular disease and mental disorders such as anxiety and depression.

Diana Beresford-Kroeger, a botanist and medical biochemist, discusses the aerosols produced by forests that fill our lungs with each of breath and boost our immune system while relaxing us with a mild narcotic effect in the myelin sheath. (Her documentary, The Call of the Forest: The Forgotten Wisdom of Trees, is available on TVO.org). For example, the Englemann spruce, found on Vancouver Island, produces two aerosols, the antibiotic compounds alpha- and beta pyrene, and Borneol acetate and limonene, compounds used in chemotherapy drugs. As she points out, the Japanese tradition of “forest-bathing”, that is, stressed city-dwellers taking long walks in the forests to relax, has a scientific basis in the aerosols released by trees. (For an analysis of studies of how forests enhance our psychological wellbeing, see Colin Capaldi’s research.)

Ironically, recreation is beneficial for humans, but creates stress on the forest itself: building trails, for example, takes up space that could be devoted to trees, and the use of these trails can lead to erosion.

The recent catastrophic flooding in Houston is a direct result of the paving over of grassy prairie lands and the loss of 65 square miles of freshwater wetlands in urban sprawl dating from the post-war years. Houston covers 600 square miles, 30% of it paved over with parking lots, shopping malls, roads, and every sort of building. Its population of 6.5 million continues to grow steadily, unhampered by environmental regulations and encouraged by low-cost housing developments. Experts in stormwater management have for decades urged measures to mitigate flooding by increasing green spaces, but have been mostly ignored. In our own backyard, the plan to reconstitute as much as possible the original waters that flow from St. Clair down to Lake Ontario via Trinity-Bellwoods is to be much applauded and supported.

From the earliest times humans have had an ambivalent attitude toward forests: at once the source of game that hunters tracked daily, as well as the source of berries, nuts and fruits that were gathered; and at the same the forest was the home of dangerous animals like wolves; an endless dark region where one could lose one’s sense of direction and location, since everywhere looked the same; and not to mention the spirits that lived in the forest, many of them malevolent, such as the Algonquian Wendego. We remember, too, that in one of the oldest works of literature, Gilgamesh (ca. 2800 BC), Enkidu is the wild man from the forests who must be tamed. Forests were seen as obstacles to both the establishment of cities and to the spread of agriculture to feed an ever-growing population. Even in Roman times a great deal of Italy and North Africa was denuded of their forests to supply food for the Empire.Click on any image to enlarge

We can see the full circle of our attitudes toward forests in Dante. First in the opening lines of The Divine Comedy is the “dark forest” (selva oscura), which contains the entrance to the underworld. In Canto XIII we encounter the horrifying Wood of the Suicides, where the souls of those who have committed suicide are turned into gnarled, thorny trees. Dante breaks off a twig and the tree screams and bleeds (43 – 44). But last, at the end of Purgatorio, we come to the selva antica, the original Garden of Eden, the paradiso terrestre. We have gone from the frightening primeval forest to the tamed forest that serves mankind (recreated by Le Nôtre at Versailles). We have today moved beyond these concepts of Nature into the world of scientific management, with a strong sense of urgency in the face of a global environment crisis.